1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device and, more particularly, to a method for manufacturing a semiconductor device capable of preventing a chemical solution from penetrating a lower layer, wherein the chemical solution is used at the time of performing a strip process and a cleaning process in the manufacturing process of the semiconductor device.
2. Discussion of Related Art
Conventionally, a strip process and a cleaning process are performed in a manufacturing process of a semiconductor device. In the strip process and the cleaning process, a chemical solution such as HF and buffered oxide etchant (BOE; the mixed solution of HF and NH4F as 100:1 or 300:1) etc., is used. However, since the solution is capable of penetrating a lower layer there below, it may damage the lower layer when used.
For example, in the manufacturing process of a flash device, shallow trench isolation (STI) process is used as a device separation process. As for performing STI process in short, a tunnel oxide film, a first poly silicon film (or pad oxide film) and a pad nitride film are deposited on a semiconductor substrate sequentially, and then, a trench is formed thereof by performing an etching process using an isolation mask. In the following step, a wall oxide film is formed inside of the trench by performing the wall oxide process. After depositing a high-density plasma (HDP) oxide film for filling the trench, a separation film is formed by performing sequentially a chemical mechanical polishing (CMP) process and the strip process for removing the pad nitride film. Next, deposition of a second poly silicon film for a floating gate is performed.
Typically, the first poly silicon film is deposited as a monolayer for simplifying process, whereby the grain boundary structure of the first poly silicon film comes to be a column direction as a whole. In such a structure, a chemical solution is subject to penetrate a tunnel oxide film and a semiconductor substrate via the grain boundary of the first poly silicon film, wherein the solution is used in the strip process and the subsequent pre-treatment cleaning process (before depositing the second poly silicon film) for removing the pad nitride film. The tunnel oxide film and the semiconductor substrate are damaged due to penetration of the chemical solution. As a result, the tunnel oxide film is damaged when a program operation and an erase operation are performed in the flash device by fouler nordheim (F-N) Tunneling method, so that the leakage current from a channel or a contact layer to a floating gate is subject to generate, resulting in the characteristic of the flash device deteriorating.
A need, therefore, exists for a method of manufacturing a semiconductor device capable of preventing the chemical solution from being penetrated into a lower layer.